Answer to Question #5426 Submitted to "Ask the Experts"

Q

I am currently involved in the evaluation of a site that is known to have reprocessed magnesium-thorium metal and disposed of the waste products on location. I am trying to find out what fraction (by activity) of alloy thorium metal is ²³⁰Th. ²³²Th and ²²⁸Th levels can be easily determined by gamma spectrometry. If we had a ²³⁰Th/²³²Th activity ratio or a range of ratios it would be a big help in screening. Any references or advice would be greatly appreciated.

A

The Multiagency Radiation Survey and Site Investigation Manual (MARSSIM) addresses the issue of using one radionuclide as a surrogate for another in Section 4.3.2 (pp. 4-4 and 4-5). It is possible to establish a surrogate relationship of ²³⁰Th to ²³²Th, so that measurements of ²³²Th can be used to infer the more difficult-to-detect ²³⁰Th. The standard practice is to collect a representative sample size (of soil) from the site and analyze each sample for both ²³⁰Th and ²³²Th to determine the ²³⁰Th to ²³²Th ratio in each sample. If the range of ratios is generally "consistent" (see MARSSIM for an example of "consistent"), then a conservative ratio is selected. That is, select a ratio that has a low probability of underestimating the ²³⁰Th present.

Note that gamma spectrometry can be used to measure ²³⁰Th, although the minimum detectable concentration for ²³⁰Th is roughly 10 times that for ²³²Th.

Eric W. Abelquist, PhD, CHP